1. Field of the Invention
The present invention relates to dielectric filters, dielectric duplexers, and communications apparatuses used mainly in the microwave band.
2. Description of the Related Art
In a known type of dielectric filter including a substantially rectangular dielectric block, the dielectric block, inner conductors, and an outer conductor constitute resonators in TEM modes, and the resonators are comb-line coupled with each other via stray capacitance generated at portions of the resonators where no conductors are formed, whereby the dielectric filter is formed.
However, in a dielectric duplexer in which an outer conductor is formed on the outer surface of such a substantially rectangular dielectric block, the dielectric block and the outer conductor cause a resonance in a mode, for example, the TE101 mode, other than the TEM mode which is the fundamental resonance mode.
FIG. 22A is a diagram showing the distribution of a magnetic field in the TE101 mode generated in the dielectric filter according to the related art, and FIG. 22B is a graph showing the attenuation characteristics of the dielectric filter.
As shown in FIG. 22B, when resonance occurs in a mode other than the fundamental mode, for example, in the TE mode, a plurality of resonance frequencies in the TE mode, in addition to the resonance frequency in the desired TEM mode, appear outside the band necessary for obtaining the desired characteristics of the filter, whereby the spurious-response characteristics of the dielectric filter are degraded.
Proposals have been made in order to avoid the effects of the TE mode. In a first proposed dielectric filter, because the frequency in the TE mode is affected by the outer dimensions of the dielectric filter, the outer dimensions are altered so as to shift the resonance frequency in the TE mode, whereby degradation of the spurious-response characteristics is avoided. In a second proposed dielectric filter, a portion of an outer conductor is cut, so that a perturbation is caused in the TE-mode resonance of the dielectric block and the outer conductor, shifting the frequency in the TE mode, whereby degradation of the spurious-response characteristics is avoided.
However, the dielectric filters according to the related art have suffered the following problems to be solved.
According to the first proposed dielectric filter, the filter must be designed for the TEM mode while also taking the effects of TE mode into consideration. In addition, because size reduction of dielectric filters is constantly desired, larger outer dimensions are inhibited. Thus, flexibility in designing filters is diminished.
In the second proposed dielectric filter, because a separate process of cutting the outer conductor is required, lead time and workload are increased, incurring additional manufacturing cost.